The invention pertains to an aqueous polymer dispersion which can be obtained by the emulsion polymerization of xcex1,xcex2-ethylenically unsaturated monomers comprising a monomer composition A and a monomer composition B, with the reactor being fed the monomer composition A continually being replenished with the monomer composition B, to a process for preparing such dispersions, to aqueous coating compositions incorporating these dispersions, and to high gloss topcoats obtained using these coating compositions.
The preparation of polymer dispersions of the aforesaid type is disclosed, int. al., in U.S. Pat. No. 3,804,881. The monomer flow charged to the reactor is continually varying in compositional content through the supply of one or more monomer flows of a different composition to the vessel from which the monomer flow is fed to the reactor. In this way particles continually varying in compositional content are obtained. A key advantage of the process described in this document is indicated to be that the thus obtained polymer dispersions are characterised by a very broad glass transition temperature range. As a result, the topcoats obtained using coating compositions prepared with these dispersions are more flexible at room temperature and have less tendency to exhibit surface tack. The adhesion of the topcoats can be improved still further by a gradual and continuous increase in the concentration of adhesion promoting monomers in monomer composition A prior to the conclusion of the polymerization reaction, which will cause a layer of adhesion promoting functional groups to form on the surface of the disperse particles.
Although coating compositions which contain dispersions prepared in this manner have very good adhesion as compared with coating compositions which do not incorporate adhesion promoting monomers, it was found that the gloss of the topcoats obtained using coating compositions containing these dispersions shows greater similarity to that of topcoats obtained using coating compositions incorporating dispersions of the core/shell type. It was further found that the simultaneous presence of adhesion promoting monomers and carboxylic groups-containing monomers required for the stability of the dispersions is attended with substantial contamination of the reactor.
It should be noted that polymer dispersions with which coating compositions for topcoats having high gloss and good adhesion can be obtained were recently proposed in U.S. Pat. No. 5,021,469. Unlike the presently proposed polymer dispersions, the known polymer dispersions are made up of particles having a core and a shell, the core consisting of a material having a glass transition temperature (Tg) of at least 40xc2x0 C. and the shell consisting of a material having a Tg of less than 70xc2x0 C. and at least 20xc2x0 C. below the Tg of the core material. The shell contains, int. al., ethylenically unsaturated carboxylic acids and nitrogenous, adhesion promoting monomers. Unlike with the preparation of the presently proposed polymer dispersions, there is substantial contamination of the reactor during the preparation. Moreover, when using the presently proposed dispersions topcoats of higher gloss and better adhesion can be obtained.
U.S. Pat. No. 5,326,814 proposes a more specific process for preparing polymer dispersions for the aforesaid use. To improve adhesion, up to 5 wt. % of an ethyleneureido group-containing monomer and 1 to 3 wt. % of an ethylenically unsaturated carboxylic acid are added. The preparative process involves first making a pre-emulsion of all the monomers except for those having an ethyleneureido group. Next, the pre-emulsion is slowly added to the reactor, with an emulsion of the ethyleneureido group-containing monomer also being introduced into the reactor during the first part of the addition process.
The topcoats obtained using these dispersions likewise are in need of improvement when it comes to gloss and tack. Moreover, the process described in this document also has substantial contamination of the reactor. Where the text refers to glass transition temperature, Tg, what is meant is Tg as calculated in accordance with a method described by T. G. Fox in the Bulletin of the American Physical Society, Volume 1, Issue 3, p. 123 (1956). This calculation employs the constants as specified by J. Brandup and E. H. Imergut in Polymer Handbook, 2nd edition, J. Wiley and Sons, New York, pp. 139-192 (1975). (Meth)acrylate in the text stands for acrylate as well as methacrylate. (Hetero)cycloalkyl in the text stands for heterocycloalkyl as well as cycloalkyl.
The invention now provides polymer dispersions with markedly enhanced properties, notably as regards the adhesion and gloss of topcoats obtained using coating compositions containing these dispersions, without any problems being experienced during the preparation of these dispersions as a result of contamination of the reactor.
The invention consists in that in the preparation of a polymer dispersion of the known type mentioned in the opening paragraph use is made of the monomer compositions A and B, one of which has a Tg in the polymerized state of at least 40xc2x0 C., as well as at least 60xc2x0 C. higher than that of the other monomer composition in the polymerized state, and the overall monomer composition is formed by ethylenically unsaturated compounds composed of:
1) at least 80 wt. % of one or more compounds selected from the group of alkenyl aromatic monomer, acrylonitrile, an alkyl, (hetero)cycloalkyl or aralkyl ester of acrylic acid and methacrylic acid having 4-22 C-atoms, acrylamide and methacrylamide, acrylamide and methacrylamide N-substituted with an alkyl, cycloalkyl or aralkyl group having 1 to 18 carbon atoms, vinyl acetate, and vinyl versatate, with up to 25 wt. % of these compounds optionally containing a second functional group capable of reacting with an appropriate curing agent,
2) 0.1 to 5 wt. % of an ethylenically unsaturated carboxylic acid, and
3) 0.1 to 5 wt. % of a nitrogenous, adhesion promoting copolymerizable monomer, with the proviso that at least 60 wt. % of the carboxylic acid is incorporated into the monomer composition holding less than 40 wt. % of the adhesion promoting comonomer, with the mass flow to the reactor of monomers of the composition containing monomer composition A, which is larger by a factor of 1.1 to 11, proceeding simultaneously with the mass flow of monomers of monomer composition B to the composition containing monomer composition A.
The invention also pertains to a process for preparing aqueous polymer dispersions of the aforesaid composition by subjecting a monomer composition A which is continually replenished with a monomer composition B to an emulsion polymerization, with the Tg of one of the monomer compositions in the polymerized state being at least 40xc2x0 C., as well as at least 60xc2x0 C. higher than the Tg of the other monomer composition in the polymerized state, with the overall monomer composition being formed by ethylenically unsaturated compounds composed of:
1) at least 80 wt. % of one or more compounds selected from the group of alkenyl aromatic monomer, acrylonitrile, an alkyl, (hetero)cycloalkyl or aralkyl ester of acrylic acid and methacrylic acid having 4 to 22 C-atoms, acrylamide and methacrylamide, acrylamide and methacrylamide N-substituted with an alkyl, cycloalkyl or aralkyl group having 1 to 18 carbon atoms, vinyl acetate, and vinyl versatate, with up to 25 wt. % of these compounds optionally containing a second functional group capable of reacting with an appropriate curing agent,
2) 0.1 to 5 wt. % of an ethylenically unsaturated carboxylic acid, and
3) 0.1 to 5 wt. % of a nitrogenous, adhesion promoting, copolymerisable monomer, with the proviso that at least 60 wt. % of the carboxylic acid is incorporated into the monomer composition holding less than 40 wt. % of the adhesion promoting comonomer, with the mass flow to the reactor of monomers of the composition containing monomer composition A, which is larger by a factor of 1.1 to 11, proceeding simultaneously with the mass flow of monomers of monomer composition B to the composition containing monomer composition A.
By xe2x80x9cmass flow to the reactor of monomers of the composition containing monomer composition Axe2x80x9d is meant according to the invention the amount of mass (A+B) fed to the reactor per unit of time. By xe2x80x9cmass flow of monomers of monomer composition B to the composition containing monomer composition Axe2x80x9d is meant the amount of mass of monomer composition B introduced into the tank holding the composition containing monomer composition A per unit of time. Monomer composition B can be charged from a single tank into the one holding monomer composition A. Alternatively, the monomers of composition B can be charged into the tank holding monomer composition A from tanks holding different compositions rather than from a single tank. In that case, composition B is first formed in situ in the tank holding monomer composition A. The mass flow of the monomer composition charged to the reactor is up to 11 times higher than the mass flow of monomer composition B to the tank in which monomer composition A is present. In this process, preference is given to a mass flow of up to 5 times higher than the mass flow of monomer composition B to the tank. Optimum results were obtained with a mass flow to the reactor twice as high as the mass flow of monomer composition B to the tank holding the composition containing monomer composition A.
The monomer composition having a Tg in the polymerized state of at least 40xc2x0 C. is essentially composed of monomers where the Tg of the homopolymers is at least 80xc2x0 C., such as styrene, xcex1-methyl styrene, acrylonitrile, and methacrylonitrile, methyl, sopropyl, tert.butyl, and cyclohexyl methacrylate or vinyl chloride.
Examples of comparatively soft monomers where the Tg of the homopolymers generally is below 80xc2x0 C. and preferably below 40xc2x0 C. are acrylate esters of the formula CH2xe2x95x90C(R1)xe2x80x94C(O)xe2x80x94OR2, wherein R1 stands for a hydrogen atom or a methyl group and R2 stands for an alkyl or cycloalkyl group having at least 2 and not more than 18 carbon atoms. Examples include n-butyl acrylate, sec.butyl methacrylate, 2-ethylhexyl methacrylate, iso-octyl acrylate, 2-ethylhexyl acrylate, n-octyl acrylate, nonyl acrylate, dicyclopentenyloxyethyl methacrylate, isodecyl acrylate, lauryl methacrylate, and stearyl methacrylate. Examples of aralkyl esters are 2-phenylethyl methacrylate and 3-phenylpropyl methacrylate. Examples of heterocyclic alkyl esters of acrylic and methacrylic acid, respectively, are furfuryl methacrylate and tetrahydrofurfuryl acrylate.
Examples of other suitable monomers are N-substituted compounds of acrylamide and methacrylamide such as N-tert.butyl acrylamide, N-tert.hexyl acrylamide, N-tert.octyl acrylamide, N-(1.5-dimethyl-1-ethyl)hexyl acrylamide, N-(1,1-dimethyl-2-phenyl)ethyl acrylamide, N-cyclohexyl acrylamide, N-(1-methylbutyl)acrylamide, N-ethyl acrylamide, N-ethylhexyl acrylamide, N-butyl methacrylamide, and N-cyclohexyl methacrylamide.
Also, functional groups such as hydroxyl, amino, epoxy, and carbonyl may be present, or ethylenically unsaturated groups such as are present in the reaction product of an unsaturated fatty acid with glycidyl methacrylate and dicyclopentadienyl acrylate.
Examples of hydroxy-functional monomers are 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, and hydroxybutyl acrylate. The hydroxy-functional monomers can be cross-linked with polyisocyanates which may be blocked or not, melamines, and urea resins.
Examples of amino-functional monomers are N,N-dimethyl aminoethyl methacrylate, N,N-dimethyl aminoethyl acrylate, and N-tert.butyl aminoethyl methacrylate.
The amino-functional monomers can be cross-linked with epoxy-functional compounds, polyisocyanates which may be blocked or not, and polycarboxylic acids.
One example of an epoxy-functional monomer is glycidyl methacrylate. This monomer can be cross-linked with di- and multifunctional compounds having a carboxyl, hydroxyl and/or amino function.
Examples of carbonyl-functional monomers are diacetone acrylamide and acetoacetoxyethyl methacrylate. They can be cross-linked with hydrazides and amines.
Examples of compounds where the Tg of the homopolymers is at least 80xc2x0 C. are alkenyl aromatic monomers such as styrene, o-methyl styrene, p-methyl styrene, o,p-dimethyl styrene, o,p-diethyl styrene, p-chlorostyrene, isopropyl styrene, t-butyl styrene, o-methyl-p-isopropyl styrene, o,p-dichlorostyrene, and mixtures thereof.
Also, cross-linking agents having two or more, say, two to six, ethylenically unsaturated groups per molecule may be present, such as triallyl cyanurate, vinyl or allyl acrylate or methacrylate, diol diacrylates and diol dimethacrylates, and methylene bisacrylamide or methylene bismethacrylamide. The quantity of these cross-linking agents used generally ranges from 0.01 to 5 wt. %. The presence of a small quantity of cross-linking agent can have a beneficial effect on the film""s hardness and the adhesion properties.
The weight ratio of monomer composition A to monomer composition B can vary within a wide range, but is generally selected in the range of 1:10 to 10:1. As a rule, preference is given to a composition where the monomer composition which has the highest Tg in the polymerized state makes up 10 to 50 wt. % of the polymer dispersion, calculated on the weight of the polymer. The ethylenically unsaturated carboxylic acid makes up at least 0.1 to 5 wt. % of the overall monomer composition. Preferably, at least 60 wt. % thereof is incorporated into monomer composition A. Of the adhesion promoting monomer preferably at least 60 wt. % is present in monomer composition B. Examples of suitable ethylenically unsaturated carboxylic acids are acrylic acid, methacrylic acid, maleic acid, fumaric acid, and itaconic acid.
The monomer composition incorporating more than 60 wt. % of the present carboxylic acid preferably contains 65 to 100 wt. % of the overall quantity of carboxylic acid. To enhance the stability of the obtained dispersions during the polymerization process it may be advantageous for up to 20% of the carboxylic acid to be present in the salt form through neutralisation with ammonia, inorganic bases such as alkali hydroxides, e.g., lithium hydroxide, potassium hydroxide or sodium hydroxide, or organic amines, such as N,N-dimethyl ethanolamine, N,N-diethyl ethanolamine, triethyl amine, and morpholine.
Optionally, in addition to the ethylenically unsaturated carboxylic acids other acid, non-carboxylic groups-containing monomers polymerizable under the influence of radicals may be present. As examples may be mentioned 2-acrylamido-2-methylpropane sulphonic acid or the alkali, ammonia or amine salt thereof and the sodium salt of the adduct of allylglycidyl ether to sodium bisulphite.
The monomer composition incorporating more than 60 wt. % of the adhesion promoting monomer preferably contains over 65 wt. % thereof, with optimum results being obtainable at a percentage in the range of 80 to 100 wt. %. Suitable adhesion promoting monomers generally are monomers polymerisable under the influence of radicals and containing amino, ureido or N-heterocyclic groups. Examples of such monomers are dimethyl aminoethyl(meth)acrylate, diethyl aminoethyl(meth)acrylate, 3-dimethylamino-2,2-dimethylpropyl- 1-(meth)acrylate, N-dimethylaminomethyl(meth)acrylamide, N-(4-morpholinomethyl) (meth)acrylamide, vinyl imidazole, and vinyl pyrrolidone. Further mention may be made of compounds having a pyrrolidine, piperidine, morpholine, piperazine, imidazole, pyrrolidin-(2)-one or imidazolidin-2-one (ethylene urea) ring. In this case preference is given to ureido-functional monomers such as N-(2-methacryloxyethyl)ethylene urea, 1-(2-(3-allyloxy-2-hydroxypropylamino) ethyl)-imidazolidin-2-one, and 2-ethyleneureido-ethyl methacrylate.
To promote film forming at lower temperatures also compounds may be incorporated which enter into a chemical combination with other monomeric, difunctional compounds at low temperature, resulting in the polymer being cross-linked.
Examples of such monomers are acrolein, methacrolein, diacetone acrylamide, diacetone methacrylamide, and vinyl acetoacetate. These compounds generally make up 1 to 4 wt. % of the overall monomer composition. At low temperature and in an aqueous medium they react with hydrazides of aliphatic dicarboxylic acids of which 0.3 to 1, preferably 0.4 to 0.8 mole is present per mole of the difunctional compound. Examples of suitable dihydrazides are oxalic dihydrazide, glutaric dihydrazide, adipic dihydrazide, sebacic dihydrazide, maleic dihydrazide, fumaric dihydrazide and/or itaconic dihydrazide.
Within the framework of the invention preference is given to compositions at least 80 wt. % of which is made up of styrene, methyl methacrylate, 2-ethylhexyl acrylate and/or butyl acrylate.
Optimum results are generally obtained when the composition of the monomers forming the polymer dispersion is such that the Tg of the copolymer thereof is in the range of 10 to 40xc2x0 C. Further, preference is given to a polymer dispersion where the concentration of ethylenically unsaturated carboxylic acid in composition A is at least twice as high as in composition B.
In actual practice, the preparation of the polymer dispersions according to the present invention will generally proceed as follows. In a first process step the use of batchwise emulsion polymerization results in so-called seed particles being formed. The number of formed seed particles is determined essentially by the temperature during polymerization and by the quantity of emulsifier and initiator used in this process. In a subsequent process step more monomer mixture is added, and the seed particles grow to become the final particles of the dispersion. The size of these particles usually does not exceed 250 nm, but preferably is  less than 150 nm.
The emulsion polymerization is carried out using a radical initiator such as alkali or ammonium persulphate, bis(2-ethylhexyl) peroxydicarbonate, di-n-butyl peroxydicarbonate, t-butyl perpivalate, t-butyl hydroperoxide, cumene hydroperoxide, dibenzoyl peroxide, dilauroyl peroxide, 2,2xe2x80x2-azobisisobutyronitrile, and 2,2xe2x80x2-azobis-2-methyl butyronitrile. Suitable reducing agents which are used in combination with, e.g., a persulphate or a hydroperoxide include: ascorbic acid, sodium formaldehyde sulphoxylate, thiosulphates, disulphates, hydrosulphates, water-soluble amines, such as diethylene triamine, triethylene tetraamine, tetraethylene pentamine, N,Nxe2x80x2-dimethyl ethanolamine, N,N-diethyl ethanolamine, and reducing salts, such as cobalt, iron, nickel, and copper sulphate. If so desired, a chain-length regulator, e.g., mercaptoethanol, n-octyl mercaptan, dodecyl mercaptan or 3-mercaptopropionic acid may be employed.
The copolymerization of the monomer mixtures is generally carried out under atmospheric pressure at a temperature of 40-100xc2x0 C., preferably 60-90xc2x0 C., in an atmosphere of an inert gas, such as nitrogen. If so desired, however, it is also possible to carry out the copolymerization under elevated pressure and at a temperature of 40-100xc2x0 C. or higher.
As a rule, the monomer concentration of the two monomer compositions is selected such that the solids content of the polymer dispersion is in the range of 25 to 60 wt. %. The monomers can be fed as such, but usually a pre-emulsion is employed.
When preparing the emulsion usually an emulsifier is employed. Preferably, the emulsifiers used in the emulsion polymerization are of an anionic or non-ionic nature. Examples of anionic emulsifiers are: potassium laurate, potassium stearate, potassium oleate, sodium decyl sulphate, sodium dodecyl sulphate, and sodium rosinate. Examples of non-ionic emulsifiers are: linear and branched alkyl and alkylaryl polyethylene glycol ethers and thioethers and linear and branched alkyl and alkylaryl polypropylene glycol ethers and thioethers, alkylphenoxypoly(ethylenoxy)ethanols such as the adduct of 1 mole of nonylphenol to 5-50 moles of ethylene oxide, or the alkali salt or ammonium salt of the sulphate or the phosphate of said adduct.
The dispersions according to the invention are pre-eminently suited to be incorporated into an aqueous coating composition. These coating compositions can be hardened by physical drying. Curing can also be carried out in a different manner, when the addition polymer contains hydroxyl groups and the aqueous dispersion contains a curing agent which reacts with hydroxyl groups. Suitable curing agents include N-methylol- and/or N-methylol ether groups-containing aminoplasts obtained by reacting an aldehyde, e.g., formaldehyde, with an amino groups- or amido groups-containing compound, such as melamine, urea, N,Nxe2x80x2-ethylene urea, dicyanodiamide, and benzoguanamine. The resulting compounds preferably are wholly or partially etherified with alcohols having 1 to 6 carbon atoms, such as methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, amyl alcohol, hexanol, or mixtures thereof. Especially favourable results can be obtained when use is made of a methylol melamine having 4 to 6 methyl groups per molecule of melamine, with at least 3 methylol groups being etherified with butanol, or a condensation product of formaldehyde and N,Nxe2x80x2-ethylene diurea etherified with butanol. Other suitable curing agents are, e.g., water-dispersible polyisocyanates which may be blocked or not, such as a methylethyl ketoxime-blocked, isocyanate group-containing adduct of a polyisocyanate to a hydroxycarboxylic acid, e.g., dimethylol propionic acid.
The dispersions according to the invention can be applied to a substrate in any manner desired, e.g., by means of rolling, spraying, brushing, sprinkling, flow coating, dipping, (electrostatic) spraying, or coating by electrophoresis. Suitable substrates include those of wood, metal, paper, hardboard and softboard, concrete, stone, masonry, glass, ceramic material, and synthetic material. Curing can be carried out at ambient temperature or, optionally, at an elevated temperature to reduce the curing time. If so desired, the composition can be baked at higher temperatures, e.g., of between 60 and 160xc2x0 C., in a drying oven for 10 to 60 minutes.
The invention will be further illustrated below with reference to the following examples, which are submitted for a better understanding of the invention only. They are not to be construed as being limiting in any manner whatsoever. Unless otherwise specified, all parts in the examples and comparative examples are parts by weight.
The viscosity in the examples below was determined using a Brookfield LVT viscometer with No. 3 spindle and 60 revolutions per minute (rpm) at 20xc2x0 C.
The solids content, SC, was determined in accordance with ISO 3251.
The gloss was determined with the aid of a BYK Haze-gloss 4601 in accordance with ISO 2813.
The particle size was determined by means of dynamic light scattering using a Malvern Autosizer Lo-C. The contamination of the reactor was measured by filtering the dispersion through a 250 mesh (about 60 xcexcm) filter gauze, and drying and weighing the residue. The percentage by weight is calculated on the overall dispersion.
Sticking together (blocking) was measured using a BYK Gardner Blocking Tester (as per Rohm and Haas). This test apparatus was used in a climatisation room to measure, at room temperature and a relative humidity (RH) of 50%, the force required to separate two panels first aged for 6 days and then placed with the paint surfaces face to face. To this end the coating composition to be tested was applied with an automatic film applicator (K Control Coater, RI Print-Coat Instrument, Ltd.) under standard conditions (23xc2x0 C. and a relative atmospheric humidity of 50% ) as 125 xcexcm thick films onto a number of white chart forms (Leneta). After 6 days of ageing at 23xc2x0 C. and 50% RH small rectangles of 25xc3x9750 mm were cut out. The side of these rectangles not covered with coating composition was then taped to glass slides with double-face adhesive tape. A BYK Gardner centering device was used to stack the slides two by two, with the coating compositions placed face to face, after which a load weight of 1000 g was placed on the slides for 2 hours. Next, the slides were arranged in the Blocking Tester, and measurements were carried out with load weights of 0.2500 g and 5000 g. The tables below list the force required to separate the films for the different coating compositions.
The hardness according to Kxc3x6nig was determined in accordance with DIN 5157. Wet adhesion was measured as follows. First, a wooden panel was primed with a filler which gives a very flat surface, after which a dark high-gloss paint based on an alkyd resin was applied. The panel was dried for at least 1 month at 23xc2x0 C. and 50% RH. Half of the panel was sandpapered with 120 grain sandpaper. Next, the coating composition was applied onto the entire panel with a 150 xcexcm applicator blade. The film was dried for 1 week at 23xc2x0 C. and 50% RH. A pad of Kleenex tissue soaked with water was applied to the film. Subsequently, a check pattern was made in the two halves using an xe2x80x9cISO-ASTM-DIN-NF crosscut.xe2x80x9d To this pattern 38 mm wide transparent filament tape (3M, ref. 898F) was stuck, which was quickly pulled loose at a 45xc2x0 angle without tearing. The result is given on a scale of 0 (no adhesion) to 5 (perfect adhesion without the film coming loose).